The idea that all animals are made of cells was first proposed by Theodor Schwann in 1839. He was a German physiologist who, along with Matthias Schleiden, formulated the cell theory.

The cell theory states that:

1. All living organisms are composed of one or more cells.

2. The cell is the basic unit of structure and organization in organisms.

3. All cells arise from pre-existing cells.

While Schleiden focused on plants, Schwann extended the concept to include animals. His observations of animal tissues, particularly the similarities between their cells and plant cells, led him to conclude that all living things are made of cells.